Field of the Invention
The present invention relates to a method and apparatus for monitoring hemodynamics in the human body, and more particularly to a system using non-invasive measurements including phonocardiography to monitor physiological parameters related to hemodynamics.
Description of the Background
Determination of central fluid and blood volume and monitoring of hemodynamics is useful in a wide variety of medical contexts including physical performance monitoring, the assessment, prevention and treatment of dehydration, fatigue alleviation, wound management, fluid resuscitation, detection of cardiac abnormalities, monitoring of cardiac function, as well as in the diagnosis and management of critical illness.
One area of particular interest is the ability to non-invasively detect disorders related to central fluid and central blood volume. Existing techniques for blood volume and stroke volume determination typically provide useful information in tightly controlled environments such as laboratories or medical facilities. However, these techniques often require a degree of ‘invasiveness’ and suffer when applied outside of a tightly controlled environment. As an example, central venous pressure, long considered to be the gold standard in monitoring central fluid volume and guiding fluid resuscitation, requires a highly invasive central venous catheter and thus is not suitable outside of controlled medical facilities. A non-invasive methodology to robustly and reliably determine blood volume and cardiac output in a variety of settings is needed since existing techniques are not practical.
Additionally, a non-invasive monitoring system that can be embedded in an article of clothing would be highly desirable and useful given the variety of situations where physiologic and biometric monitoring can be employed. The need to integrate a system into a wearable device places restrictions on the size, power, and weight. Again, current technologies for monitoring of hemodynamics non-invasively are not particularly suitable in this regard and typically do not provide accurate results.
Phonocardiography
During the cardiac cycle, blood moves through the different chambers of the heart due to pressure differences between the chambers and accompanying vessels. Valves in the heart prevent the backflow of blood and open and close based on pressure differences. Certain heart sounds occur primarily due to the closure of the heart valves. This snapping action produces sound waves which travel through the blood and thoracic tissue to the surface where auscultation devices are used.
Phonocardiography techniques evaluate and examine the sounds which the heart produces during the course of the cardiac cycle. The phonocardiograph waveform can be determined using transducer(s) attached or mounted on the surface of the body. In healthy subjects the prominent features of the phonocardiogram are the S1 and S2 components (“lub” and “dub”, respectively) which mark the beginning and end of the ventricular contraction. The S1 component of the phonocardiogram marks the closure of the mitral and tricuspid valves and the S2 component marks the closure of the aortic and pulmonic valves.
The phonocardiogram wave form may also display additional components (e.g., murmurs, rubs and gallops) besides the S1 and S2 components and their presence may be benign or serve as indicators of abnormalities.
There is a long and robust history of using hearts sounds and phonocardiography to diagnose cardiac aberrancies and specific valvular abnormalities, dating back to the late 1940s. Over the next several decades Phonocardiography evolved to be an effective and widely used technique in clinical practice before the advent of other methods to evaluate the heart and its function such as ultrasound and coronary angiography, but its prevalence has diminished as a consequence of several issues. As noted by Sprague, in “The Clinical Value of Phonocardiography” (1954), the clinical value of phonocardiography suffered from the limitation of then existing analog electronics. Interest was regained in 1990s with the advent more advanced digital signal processing techniques to the analysis of heart sounds. See, Brusco and Nazeran, Development of an Intelligent PDA-based Wearable Digital Phonocardiograph, Engineering in Medicine and Biology 27th Annual Conference (2005). Examples of prior art phonocardiography systems include:
United States Patent Application 20130338724 by Joo et al. published Dec. 19, 2013 shows a pulse detection system that measures two parameters such as phonocardiogram (PCG) signals, electrocardiogram (ECG) signals, patient impedance signals, piezoelectric signals, and accelerometer signals, and analyzes the combined signals for features indicative of the presence of a cardiac pulse. The medical device is wearable (electrodes).
PCT Publication number WO2013184315 filed May 15, 2013 by Bedingham et al. (3M) shows a phonocardiogram adapted to combine an acoustic signal and the electrical signal detected over a plurality of cardiac cycles of the heart.
U.S. Pat. No. 5,012,815 to Bennett, Jr. et al. issued May 7, 1991 shows a dynamic spectral phonocardiograph that summarizes time-dependent changes in the heart sounds based on a Fourier transform of heart sounds as a function of time.
U.S. Pat. No. 5,812,678 to Scalise et al. issued Sep. 22, 1998 shows a noise-canceling phonocardiographic sound monitoring system.
U.S. Pat. No. 5,638,823 to Akay et al. (Rutgers) issued Jun. 17, 1997 shows a system and method for noninvasive detection of coronary artery disease by phonogrammetric analyses.
U.S. Pat. No. 8,478,391 to Scheiner et al. (Cardiac Pacemakers, Inc.) issued Jul. 2, 2013 and related U.S. Pat. No. 8,663,123 issued Mar. 4, 2014 show an apparatus and method for outputting heart sounds using an implantable system that transmits to an external system.
United States Patent Application 20100094152 by Semmlow published Apr. 15, 2010 shows a phonocardiographic system and method for acoustic detection of coronary artery disease.
U.S. Pat. No. 6,149,595 to Seitz et al. issued Nov. 21, 2000 shows a noninvasive apparatus and method for the determination of cardiac valve function.
U.S. Pat. No. 7,666,144 to Cohen et al. (Michigan State University) issued Feb. 23, 2010 shows a method and apparatus for determining proportional cardiac output (CO), absolute left atrial pressure (LAP), and/or other important hemodynamic variables from a contour of an RVP waveform attained by a phonocardiogram.
U.S. Pat. No. 8,290,577 to Brooks et al. issued Oct. 16, 2012 shows a method and apparatus for enhanced fiducial point determination and non-invasive hemodynamic parameter determination from phonocardiography.
United States Patent Application 20050222515 Polyshchuk et al. (Biosignetics Corporation) published Oct. 6, 2005 shows techniques of interpreting cardiovascular sounds with a self-referencing feature based on stored phonocardiograms.
U.S. Pat. No. 8,348,852 to Bauer et al. (Inovise Medical, Inc.) issued Jan. 8, 2013 shows heart-activity sound monitoring using an acoustic sensor resonant at S1, S2, S3, S4 heart-sound frequencies, using a wave generator to stimulate it, and observing over time any time-based differences which exist between the two as being representations of the subject's S1, S2, S3, S4 heart sounds, and from such interpreted differences, producing a utility output associated with the subject.
Studies by NASA in the 1980's included phonocardiography as one method of thoroughly evaluating astronaut health both pre- and post-flight (Bergman, Stuart, Robert Johnson, and G. Wyckliffe Hoffler. “Evaluation of the Electromechanical Properties of the Cardiovascular System After Prolonged Weightlessness.” In Biomedical Results from Skylab, 351-365, 1977). Buried within this data a correlation between parameters identified via phonocardiography and volume status can be found. During Lower Body Negative Pressure testing of the astronauts, a significant difference in the ventricular ejection time as indicated by the time between S1 and S2 heart sounds were observed between normal conditions and Lower Body Negative Pressure conditions.
Unfortunately the challenges with phonocardiography based techniques included a need for highly sensitive equipment and a need for exquisitely controlled testing conditions. As a result, phonocardiography has more recently fallen out of clinical practice in favor of these other techniques. It is important to note, however, that these current techniques require bulky, expensive, and high power equipment along with a highly trained user.
Hemodynamics and Blood Volume Status
Ventricular contraction in the cardiac cycle results in the movement of blood from the heart into the circulatory system. The QRS complex is a known feature of the electrocardiogram and can be used as a reference for the cardiac cycle. Most notably, the R wave is a prominent upward deflection in the signal. The start and end of ventricular contraction is closely and physically linked with the S1 and S2 heart sounds. Assessment of blood volume and cardiac output may be performed using heart sounds.
In order to validate that events within a cardiac cycle are affected by dehydration, the inventors herein first conducted proof-of-principle testing to evaluate the effect that dehydration would have on Phonocardiographic Systolic Time Intervals (PSTI; i.e. S1-S2 time divided by heart beat period). Body weight and PCG waveforms (applying an off the shelf microphone) were collected on a human test subject before and during an intensive workout routine. Gross physiological parameters from the test including weight loss, hear rate, blood pressure, and blood oxygenation for euvolemic and dehydrated states are shown in FIG. 7. Data reduction from phonocardiography readings was used to determine the PSTI for each state over a 1 minute period. The PSTI for a normally euhydrated state, dehydrated state at up to 1.7% loss in body mass, and recovered state is shown in FIG. 7. The change in S1-S2 time, normalized for heart rate, is significantly higher in the dehydrated state, indicating a change in the hemodynamics associated with dehydration. This initial testing demonstrated high sensitivity to even mild dehydration—over five times more sensitive than standard clinical assessment. In the testing data, the PSTI is determined using time based features corresponding to the S1 and S2 from the phonocardiography transducer normalized by the time between the R wave and subsequent R wave in the electrocardiogram. Similar information may be determined using the S1 and subsequent S1 in the phonocardiogram for normalization.